This invention relates to an optical modulation device for modulating light emitted from a light source in various optical apparatus such as an image projection apparatus and a printer and an image display apparatus which uses an optical modulation device.
As a method of raising the resolution of an image in various image display apparatus such as a projector and a printer, a method wherein a flux of light from a one-dimensional image displaying optical modulation device is scanned by a light scanning mechanism and projected on an image forming mechanism to form a two-dimensional image is known and disclosed, for example, in U.S. Pat. No. 5,982,553.
As one of such one-dimensional optical modulation devices, a GLV (Grating Light Valve) developed by Silicon Light Machines (SLM), a company of United States, is known and disclosed, for example, in U.S. Pat. Nos. 3,164,824 or 5,841,579.
The GLV employs a diffraction grating of the phase reflection type which makes use diffraction of light. As an example of the GLV, a GLV of a ribbon element type configuration has been proposed and is shown in FIG. 18. Referring to FIG. 18, the GLV of the type mentioned includes, for example, three first surface elements 30a and three second surface elements 30b disposed alternately. Each of the first surface elements 30a is formed from a movable ribbon while each of the second surface elements 30b is formed from a fixed ribbon.
In the GLV having the configuration described above, if a suitable voltage is applied between a common electrode 33 on the substrate (not shown) side and the first surface elements 30a, then the first surface elements 30a are deformed to move by a suitable amount toward the substrate side thereby to form a diffraction grating to incoming light.
Since the first and second surface elements in the GLV have a width of, for example, approximately 25 μm and a length of, for example, approximately 200 μm and hence are very small in size, the GLV has an advantage that a high speed switching movement of the first surface elements, that is, of the movable ribbons, is possible. Further, since display of a broadband width can be realized, that is, since a high degree of modulation of the variation of the light amount can be realized, by changing the reflection factor of diffracted light in accordance with the distance of movement of the movable ribbons, a small-sized image display apparatus having a high resolution and a low operation voltage can be provided using the GLV.
In a one-dimensional optical modulation device such as a GLV as described above, when compared with a two-dimensional optical modulation device such as a DMD (Digital Micromirror Device: Texas Instruments) which uses a liquid crystal panel or a micromirror unit, the illumination light density necessary to obtain an equal luminance is, for example, in display of 1,080×1,902 pixels, as high as approximately 1,920 times. More particularly, in order to achieve a high luminance of 10,000 [lm] (lumen) as in an application as a projector for a theater, the laser illumination power to an optical modulation device which uses the GLV or the like is very high and approximately 50 to 100 W, and if this is condensed into a linear beam of approximately 25 μm wide (a spot diameter of 1/e2 of the light intensity) on the surface of a ribbon of the GLV, then a void or hillock is produced on the GLV ribbon and degrades the GLV ribbon, resulting in a problem that the dark level is deteriorated and, in the worst case, the GLV is electrically broken and does not function any more.